Repairing phase shift structures on phase shift masks (PSMs) presents formidable challenges. Requirements for PSM repair go far beyond those needed for conventional chrome masks, where pinholes are made opaque and pindots are removed from the surface with what are essentially two-dimensional processes. Defects in the phase material, such as inclusions, chips, or excess material must be repaired to leave optically correct, three-dimensional structures. Unavailability of equipment and methods to handle phase shift defects is one of the greatest barriers to the routine commercial acceptance of phase shift mask technology.
In this paper we will demonstrate multiple exposure (or "vote-taking") lithography methods for eliminating the effect of PSM defects on the wafer. Two (or more) exposures are made for a single wafer lithography level using multiple masks. The probability of a random defect occurring in the same location on separate masks is virtually nil. Therefore, dark defects - the consequence of defective phase structures - caused by any single exposure are overprinted by the other exposure(s).
This technique also can provide relief from other potential error sources unique to PSMs. Wafer CD uniformity can be improved. Dimensional variations caused by transmission differences between phase and non-phase features will be averaged out by reversing the phase sense among the masks. PSM layout flexibility is enhanced. Normally, abrupt phase transitions (between 0° and 180°) within a bright region will print a dark band. By eliminating this "defect" through complementary exposures, such phase transitions can be used to help meet the boundary conditions imposed by a PSM layout.